The present invention relates to an electric supercharger using an electric motor to drive a compressor.
In recent years, the number of vehicles mounted with an internal combustion engine such as a gasoline engine and a diesel engine provided with a supercharger such as a turbocharger has been increasing. Specifically, the number of vehicles mounted with an internal combustion engine having a small displacement for the vehicle size and equipped with a turbocharger for compensating for the power shortage due to the small displacement has been increasing. In general, a turbocharger is a mechanical device that drives a turbine by using exhaust gas of an internal combustion engine thereby driving a compressor to force extra air into the engine.
As is known to those skilled in the art, a response of supercharging tends to be delayed when the internal combustion engine is accelerated rapidly from a low speed and hence the flow rate of exhaust gas is low. To improve the response, a turbocharger provided with a variable nozzle in the turbine is used widely. To further improve the response, a turbocharger has been developed which is provided in addition to a mechanical turbocharger and uses an electric motor to drive a compressor.
There are known some electric supercharger or electric turbocharger in which a mechanical turbocharger is combined with an electric motor. The motor assists a turbine to drive a compressor temporally and compensates for the rotation of the compressor when the delayed response or the turbo lag is estimated. The motor also performs the supercharging continuously at a low rotational speed of the internal combustion engine. In other cases, a mechanical turbocharger and an electric supercharger are individually provided in series. The electric supercharger is operated temporarily to prevent the delayed response of supercharging, or it is operated continuously while the internal combustion engine is running at a low speed.
Japanese Patent Application Publication No. 2007-321675 (JP '675) discloses an electric turbocharger incorporating a mechanical turbocharger and an electric motor. The electric turbocharger described in the above Publication has a shaft having at one end thereof a compressor wheel and at the other end thereof a turbine wheel, and a rotor for an electric motor that is mounted on the shaft on the side thereof where the compressor wheel is mounted. In the electric turbocharger, a hole is formed in the shaft adjacently to the turbine wheel so as to reduce the sectional area of the thermal conductive passage of the shaft through which the heat of the turbine wheel is transmitted therefrom through the shaft to the rotor. Such structure suppresses the rise of temperature in the rotor without affecting the rigidity of the shaft.
Japanese Patent Application Publication No. 2005-320967 (JP '967) discloses an electric supercharger. The electric supercharger has therein a shaft, a compressor and a compressor housing disposed on one end of the shaft, and an electric motor including a rotor, a stator and a motor housing disposed on the other end of the shaft. The electric supercharger is used for compression and delivery of cryogenic temperature liquid, for example, delivery of liquefied natural gas (LNG) and releasing gas in air decomposition. It is undesirable for any part of the shaft including a bearing which is located adjacent to the electric motor to be cooled down to a cryogenic temperature due to rendering the lubricant in the bearing unserviceable. Furthermore, a permanent magnet of the rotor, if cooled down to a cryogenic temperature, being demagnetized is also undesirable. Additionally, a coil of the stator cooled similarly becomes susceptible to breakage by vibration because the material of the coil becomes brittle under a cryogenic temperature. In order to solve these problems, the electric supercharger described in the above-cited Publication has a separation wall made of an insulating material between the compressor housing and the motor housing.
The electric supercharger generally includes an electric motor unit having a shaft, a rotor, a stator and a motor housing, a compressor unit including a compressor housing and a compressor impeller mounted on the shaft at one end thereof, a bearing supporting the one end of the shaft, and a bearing supporting the other end of the shaft. The bearings are provided in the motor housing. As described in JP '967, the lubricating substance of the lubricant for the bearings fails to function as a lubricant also when the lubricant is subjected to an excessively high temperature, as well as to a very low temperature. Therefore, the heat to be transmitted to the bearings needs to be suppressed so that the temperature of the bearings does not exceed a predetermined high temperature. The coil of the electric motor generates a large amount of heat, especially in case that the internal combustion engine running at a low speed is supercharged continuously. The heat may raise the temperature of the bearings to a harmful temperature.
In the electric supercharger or electric turbocharger disclosed in JP '675, high temperature of exhaust gas can be suppressed to transmit to the rotor through the thermal conductive passage including the turbine and the shaft. However, it is found in the above Publication neither about a thermal conductive passage through which the heat generated by the coils of the electric motor is transmitted from the motor housing to the bearing, nor the indication that the conductive passage suppresses transmission of heat from the coils.
The electric supercharger disclosed in JP '967 is disadvantageous in that the separation wall made of a thermal insulation material and provided between the compressor housing and the motor housing increases the number of the parts of the electric supercharger. As with JP '675, JP '967 discloses nothing about a thermal conductive passage through which the heat generated by the coils of the electric motor is transmitted from the motor housing to the bearing and the effect of the thermal conductive passage to suppress the heat transmission.
The present invention, which has been made in light of the above-identified problems, is directed to providing an electric supercharger having a compressor and an electric motor and configured so as to suppress the transmission of heat generated by coils of the electric motor through a motor housing to a bearing.